A fixed displacement pump has a theoretical fixed output of oil for each revolution of the pump, assuming the pump chambers are completely filled. The pump thus has an output proportional to the speed of the pump. An engine driven pump for a transmission will thus have a wide output range dependent on engine speed. The pump must be sized for the most demanding application, which is a low speed garage shift when high output volume is required at low speed. The pump must be filled adequately at such a low speed in order to sufficiently pressurize the transmission in order to accomplish the shift. However, sizing the pump to satisfy a low speed condition will create very large potential flow volume at high speeds. The hydraulic system feeding the pump may reach a high speed fill limit, i.e., a maximum fluid input into the pump is reached and is insufficient to keep the pump chambers full at high speeds. This will cause incomplete filling of the chambers, resulting in an air and oil combination in the chambers, potentially causing cavitation. Cavitation may cause pressure instability that interferes with transmission control valves, potentially gives rise to objectionable noise, and can cause physical damage to the pump.
Introducing pump bypass oil near a filter outlet feeding into the pump is known to create a pressure differential between the bypass oil feed into the pump and the filter outlet to thereby draw additional oil through the filter outlet from a sump, decreasing cavitation. Bypass oil is excess pressurized oil from the pump that is returned to the filter outlet without being delivered to the transmission. Bypass oil flow is controlled by a regulator valve that directs bypass oil away from the transmission when excess fluid pressure is detected at the pump outlet. Increasing the fuel efficiency of a vehicle can be accomplished, in part, by reducing the transmission pump size. A smaller pump has an associated decrease in bypass oil flow. Increasing inlet flow at high speeds is more challenging with a smaller pump.
One mechanism for increasing the volume of pumped fluid is a siphon used for draining flooded basements. Such a siphon may utilize a diffuser at a discharge end. Pressurized fluid flows through an inlet, either through an annular nozzle or a standard nozzle, creating a pressure differential to siphon fluid through an adjacent suction port.